In some connection it is important to control the alkalinity of aqueous solutions. This may be the case in chemical industrial processes where it is important to maintain alkalinity within certain limits to ensure desired reactivity or the opposite, e.g. to prevent corrosion or the like.
A specific and commercially important application is in relation to recovery of petroleum products, especially offshore, where mixtures of petroleum components, inevitably comprise some water, and in which circulation/recirculation of a glycol containing liquid is used for hydrate inhibition in the production system for gas and condensate. In such connections it is important to have control over alkalinity. This includes control of alkalinity in water-lean glycol injected into the production system and in the salt removal systems used for regeneration of water-rich glycol.
By glycol, we mean here the various forms of glycol compositions, mono-ethylene glycol (MEG) [ethane-1,2-diol], di-ethylene glycol (DEG) [2-hydroxyethoxy)ethan-2-ol], Tri-ethylene glycol (TEG) [2-[2-(2-Hydroxyethoxy)ethoxy]ethanol], and other Poly-ethylene glycols (PEG) as well as propylene glycol (PG) [1,2-propanediol]. However, precise measurement of alkalinity according to the invention provides valuable advantages for the use of a broader range of alcohol-based Thermodynamic Hydrate Inhibitors (THI), such as methanol and ethanol and also for purely aqueous solutions.
A discussion of glycol regeneration systems is given by C. A: Nazzer: “Advances in Glycol Reclamation Technology” (OTC 18010), presented at the 2006 Offshore Technology Conference, OTC, in Houston, Tex. USA, 1-4 May 2006.
Relevant background information in this technical field is also found in a publication of S. Brustad, K.-P. Løken, and J. G. Waalmann: “Hydrate Prevention using MEG instead of MeOH: Impact of experience from major Norwegian developments on technology selection for injection and recovery of MEG” (OTC 17355), presented at the OTC in Houston, Tex. USA 2-5 May, 2005.
An example of the importance of controlling the alkalinity of glycol applied for hydrate inhibition is given by O. Hagerup and S. Olsen: “Corrosion Control by pH stabilizer, Materials and Corrosion Monitoring in a 160 km Multiphase Offshore Pipeline”, CORROSION/2003, Paper No.03328, NACE, Houston, 2003.
The commonly used principle for determining the condition of such solutions comprises isolating an amount of the solution to be controlled and titrate with acid and alternative method is to allow CO2 to bubble through the solution until is it saturated therewith, whereafter pH is measured. A disadvantage with the first method is that it fails when the solution contains bases that are weaker than bicarbonate. A disadvantage with the second method is that the amount of CO2 added is not known and the pH value measured therefore does not reveal all information of interest. In addition, bubbles generate foam in the solution, making measurements difficult. Any presence of salts of (weak) organic acids, which commonly occur, will interfere with the measurements based on titration.
Object
It is thus an object of the present invention to provide a method for determining alkalinity quickly and reliably in aqueous solutions, with high accuracy, using inexpensive means.
The Present Invention
The above mentioned objects are achieved by the present invention which according to a first aspect comprises a method as defined by the appended claim 1.
According to another aspect the invention comprises an apparatus for performing the method, said apparatus being disclosed by claim 10.
According to yet another aspect the present invention concerns a method related to controlling chemistry of glycol-containing-liquid in a system for recovery of glycol as defined by claim 14, said method comprising all steps in the method according to the first aspect of the invention.
Preferred embodiments of the inventions are disclosed by the dependent claims.
By “unbonded CO2” as used herein is understood dissolved or dispersed CO2 molecules in the liquid, i.e. not in the form of HCO3− or CO32−.
While alkalinity analysis is the technical field of this invention, it should be mentioned that a pH measurement in the acidic area (negative alkalinity) does not imply that the method is irrelevant or useless. It simply implies that alkaline addition is required to bring the pH of the liquid back to the desired pH before the method according to the present invention is continued.
It should be emphasized that the term “comprises/comprising” when used in this specification is taken to specify the presence of stated features, integers, steps or components, but does not preclude the presence or addition of one or more other features, integers, steps, components or groups thereof. In the following description, for purposes of explanation and not limitation, specific details are set forth such as particular architectures, interfaces, techniques, etc. in order to provide a thorough understanding of the invention. However, it will be apparent to those skilled in the art that the invention may be practiced in other embodiments that depart from these specific details. In other instances, detailed descriptions of well-known devices and methods are omitted so as not to obscure the description of the invention with unnecessary detail.
While the method according to the present invention may be carried out as a fully manual operation, the method may also be automatized and set to run at fixed or programmable intervals.
The present invention, providing a robust, precise and repeatable process is well adapted for automatisation. FIG. 1 illustrates schematically an embodiment of an automated analyzer of the present invention, and a possible rough automation sequence process will be described hereinafter with reference to FIG. 1.
The fluid being measured is typically under pressure, and separated from the analyzer by the valve V2. The fluid to be sampled is allowed in at V2, and fills up the measurement container up to 90% of the total volume of the container. Between two measurements, the container is typically filled with a gas not containing CO2, such as nitrogen, which may have been blown through the measurement container at the end of the preceding measurement. Venting the pre-existing gas (nitrogen) may be done at V1 upon opening V2, or it may be done afterwards. For example in the case of a foaming sample, it may be an advantage not to vent at the beginning, let the sample in, let the foam be pressurized and settle for a time, then open V1 to obtain wished pressure. An alternative in the case of foam may be to connect V1 to a tube projecting further inside the measurement container so as first to let liquid out when opening V1. An alternative is also to fill up the measurement container thanks to vacuum, generated by a vacuum pump and transmitted by V8. Filling may be controlled by pressure, by a timer and/or by for example, filling level control sensors (represented here by a Level Switch High, LSH , and a Level Switch Low, LSL).
At that stage, one of the CO2 removal operations can be performed: bubbling with a non-CO2 containing gas, heating, resting, or applying vacuum.
Then, pressure and pH are measured. A fixed and known quantity of CO2 gas is let in, for example by opening V5 after a calibrated buffer volume has been filled by CO2 under pressure, preferably higher than the sample pressure. The calibrated volume is then refilled for next measurement thanks to the sequential control of valves V4 and V5. An alternative is to let CO2 in based on a timer controlling opening of an inlet valve. After CO2 has been let in, a stirrer is run for typically several minutes.
A new set of measures of pressure P and pH is taken, and alkalinity characteristics derived. Then, the container is emptied and cleaned. For example, V2 is opened and under the pressure of gas, the container expels the liquid when opening V6, and an extra flushing or drying can be performed by flowing a liquid or a gas, for example an inert gas, through V7, blowing away the remaining drops of liquid out at V6. Finally, V6 is shut, then V7.
All sensors, valve actuators and controls are driven by a control system, based on a personal computer, a Programmable Logic Controller (PLC), or any other control system as known in the art.
Basis for the Calculations
Below the basic principles behind the calculations according to the present invention are discussed.
Total alkalinity is calculated on the basis of measured pH and measured CO2 pressure according to the following equation:Total alkalinity=K*pCO2/10−pH,  Iwhere K is a constant defined by ambient temperature, measured pressure and ion concentration in the aqueous solution, as well as the presence of other solvents, while pCO2 is the difference between pressure in the container before and after CO2 addition.CCO2,d+COH−+CCO32−=(mo0−mo1)/VV  IIwhere CCO2,d is the molar concentration of CO2 dissolved in liquid, COH− is the molar concentration of OH− reacting with CO2, CCO32− is the molar concentration of CO32− reacting with CO2, mo0 is mole CO2 added, mo1 is mole CO2 in the gas at equilibrium and VVis the sampled amount (volume) of liquid.
CCO2,d is also defined by:CCO2,d=pCO2*KH  IIIwhere pCO2 is the CO2 pressure measured in gas phase and KH is the solubility constant of CO2.
WhenCOH−+CCO32−=(mo0−mo1)/VV−CCO2,d>0  IVwhich is always the case in situations of interest for the present invention, thenTotal alkalinity=COH−+2*CCO32−  VCarbonate concentration is given by:CCO32−=Total alkalinity−(mo0−mo1)/VV+CCO2,d  VIwhile OH− concentration is given byCOH−=Total alkalinity−2*CCO32−—  VII
These formulas allow an accurate calculation of total alkalinity as well as the components thereof when performing the present invention. It is preferred to frequently calibrate the basis for the calculations by determining the constant K and the solubility constant KH of CO2 by addition of known amount of alkalinity.
In order to ensure that the sampled liquid does not contain CO2 which is not chemically bonded, it is preferred to treat the sampled liquid in one or more of the following manners prior to conducting the measurements:                bubbling the sampled amount of the aqueous liquid with N2 or other inert gases        heating the sampled amount of the aqueous liquid,        allowing the sampled amount of the aqueous liquid to rest before continuing, and        applying vacuum to the sampled amount of the aqueous liquid.        
For the purpose of said treatment, the apparatus should preferably be equipped with suitable means therefore.
By “inert gas” as used in this context is contemplated any gas not interfering with the measurements, typically N2, while principally any gas not containing CO2 could be used as inert gas.
In order to obtain reliable measurements, the amount of liquid sampled should not exceed 90% of the container volume and neither constitute less than 10% of the container volume.
To ensure that the CO2 added in step iii is present in gaseous form only, the pressure should be maintained at a level no higher than 55 bar while a pressure in the range between 3 and 10 bar is preferred. Reducing the pressure under 3 bar may result in less precise measurements while increasing the pressure above 10 bar will require inconveniently expensive equipment.
All measurements required by the method according to the present invention may be performed by standard equipment therefore.
The apparatus should preferably be approved for a pressure of at least 3 bar and more preferably at least 10 bar to allow versatility of use.
For the purpose of its calibration, the apparatus should preferably comprise means for adding defined quantities of alkalinity or acid with known concentration.
Whereas current glycol reclamation plant engineering allows for a full container skid to host alkalinity analysis, an analyzer according to the present invention would fit in a portable box. This enables a more flexible use by operators, and reduces volume required and costs.